The present invention relates to the sintering of ceramic substrates and, more particularly, relates to the expanding of alumina ceramic substrates that have become undersized after sintering.
The use of alumina ceramic substrates, usually and preferably multilayered, in electronic applications is well known as demonstrated by, for example, Anderson et al. U.S. Pat. No. 4,104,345, the disclosure of which is incorporated by reference herein. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers formed prior to lamination, which, upon sintering will become a sintered dense metal interconnection of metal based conductor.
In general, conventional alumina ceramic structures are formed from alumina ceramic greensheets which are prepared by mixing a ceramic particulate, a thermoplastic polymeric binder, plasticizers and solvents. The ceramic particulate will usually contain alumina particles plus other additives such as silica, titania, chromia, magnesia and calcia. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible greensheets. After blanking, via formation, stacking and laminating, the green sheet laminates are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate.
The electrical conductors used in formation of the alumina ceramic substrate are typically refractory metals such as molybdenum or tungsten.
A typical firing cycle for alumina ceramic substrates is to ramp up to about 900 to 950.degree. C. and hold for a predetermined time, for example 2-3 hours, depending on furnace and product characteristics, to burn off the binder and then raise the temperature to about 1600.degree. C. and hold there for about 1 to 10 hours to accomplish sintering of the alumina ceramic substrate. Thereafter, the temperature is ramped down to room temperature.
As a result of the sintering process, the alumina ceramic substrate undergoes substantial volumetric shrinkage, on the order of 17% or so. The bulk of the shrinkage occurs in the plane of the alumina ceramic substrate, the so-called X and Y dimensions, with the remainder of the shrinkage occurring in the so-called Z direction. It should be understood that the X and Y dimensions in the alumina ceramic substrate must be extremely tightly controlled so that pad-to-pad, via-to-via spacing and other dimensions are within predetermined tolerance. If an alumina ceramic substrate is found to be over tolerance, it may be refired to bring it down into tolerance. On the other hand, if the alumina ceramic substrate has been found to be under tolerance, that is, it has shrunk too much, the alumina ceramic substrate must be scrapped. Such alumina ceramic substrates are often very expensive so it is undesirable to scrap such a part.
It would be desirable to have a method to expand alumina ceramic substrates which have shrunk too much during sintering so that the alumina ceramic substrate need not be scrapped.
Accordingly, it is a purpose of the present invention to have a method to expand alumina ceramic substrates which have shrunk too much during sintering.
It is another purpose of the present invention to have such a method to expand alumina ceramic substrates which is easy to implement in a manufacturing environment.